1. Field of the Invention
The present invention relates to a display device with enhanced connection reliability between terminals on the insulating substrate and a drive circuit directly mounted thereon, and a method of manufacturing the display device, which are particularly effective in application to liquid crystal display devices.
2. Description of the Related Art
One advanced application that has been developed as a low-cost manufacturing method of display devices, such as liquid crystal display devices, is Chip-On-Grass (COG), in which a drive circuit is mounted directly on an insulating substrate. This technique directly attaches a drive circuit where Au bumps are created to terminals formed on the periphery of an insulating substrate with a conductive material such as an anisotropic conductive film (ACF). When using this technique, a power line and signal lines for driving the drive circuit are connected to the terminals (bumps) formed on the drive circuit through the ACF. Since a number of bumps are formed on the drive circuit with small pitch, the pitch of the power line or the signal lines formed on the insulating substrate is narrowed accordingly. This increases line resistance and causes defects such as malfunction of the drive circuit.
One technique for solving the above problem in conventional COG display devices is disclosed in Japanese Unexamined Patent Application Publication 11-52405 and illustrated in FIG. 2, for example. This technique covers line parts on a cell of a display element with metallic plating and/or vapor deposition. The electrical junction to a glass substrate is not covered with the metallic plating and/or vapor deposition, thereby allowing the observation of the connections to the glass substrate. Since the metallic plating and/or the vapor deposition are performed only on a power source line and a ground line to a drive circuit and a pressurizing line, it is able to reduce the resistance to the lines.
Another conventional technique is disclosed in Japanese Unexamined Patent Application Publication 2002-244151 and illustrated in FIG. 9, for example. This technique forms a plurality of terminals on a substrate and covers the terminals with a protective film having pad contact holes corresponding to the terminals. With the protective film having the pads for connection with a drive circuit, it is able to achieve good contact characteristics.
Though the above conventional techniques provide the structures of display devices with a drive circuit mounted directly on the terminals of an insulating substrate, they do not provide the structure for suppressing the increase in contact resistance due to the material of the lines and terminals of the insulating substrate. When mounting the drive circuit on the insulating substrate by the COG technique, it is difficult to form large line patterns of the power line and signal lines in the area where the drive circuit is mounted for resistance reduction, as described above. Particularly, since the power line and the signal lines transmit, to the drive circuit, input signals supplied from outside through a flexible circuit substrate and the lines and terminals on the insulating substrate, their resistance should be low. Otherwise, it causes low voltage supply capacity or voltage drop in the power line, delay in the signal lines, and so on, which leads to malfunction of the drive circuit. To prevent this, a conductive film with relatively low resistance, such as Al, is used as the material of these lines. On the other hand, lines connected to the lines in a display area of the insulating substrate where pixels are to be formed do not necessarily have such a low resistance. Thus, a conductive film with relatively high resistance, such as Cr, is used as the material of the lines.
In the case of connecting the terminal of the drive circuit and the terminal of the insulating substrate through a conductive material such as ACF, the terminal of the insulating substrate is often formed with a transparent conductive film connected to a metal material of the lines. The power line and the signal lines extending from the external flexible circuit substrate through the lines and terminals on the insulating substrate to the drive circuit are made of a low resistance material such as Al as described above. Thus, the contact resistance undesirably increases between the transparent conductive film made of Indium Tim Oxide, (ITO), SnO2, and so on and the low resistance material such as Al.